Insights from a Landmark German Study: The Bacterial Spectrum in Mare Uterine Health

A comprehensive study published in the Journal of Equine Veterinary Science in February 2024 provides the largest dataset to date on the bacterial pathogens found in the uterus of broodmares. Analysing 28,887 endometrial swab samples, the study offers a detailed picture of the microbial landscape driving uterine infection and infertility in the mare population — and its implications for how we diagnose and treat these conditions.

The Scale of the Problem

The headline finding is that 25.9% of all samples showed growth of potentially pathogenic bacteria. In a breeding population of any significant size, that figure has major implications. Roughly one in four mares is carrying a detectable uterine infection at the time of sampling — and this figure represents only what standard culture methods can detect. Dormant, persister-state bacteria are not captured by swab culture, meaning the true prevalence of uterine infection is likely considerably higher.

For breeders managing large numbers of mares, these numbers reinforce what many already observe in practice: uterine infection is not a rare complication, it is a routine challenge that must be actively managed.

The Bacterial Spectrum: What the Study Found

The study identified a clear hierarchy of pathogens in terms of prevalence, with beta-haemolytic streptococci dominating the findings by a substantial margin:

• β-haemolytic Streptococci (primarily Strep zoo): 79.7% of positive samples — by far the most prevalent pathogen. Their dominance underscores why targeted streptococcal management is central to improving mare fertility outcomes.
• Escherichia coli variatio haemolytica: 5.2% — a secondary but significant contributor, particularly relevant in mares with compromised uterine clearance.
• Escherichia coli (severe growth): 4.3% — indicating meaningful coliform involvement in a subset of cases.
• Klebsiella pneumoniae: 3.9% — a pathogen associated with more severe and treatment-resistant endometritis, often transmitted venereally.
• Candida species: 2.9% — a reminder that fungal pathogens must be considered, particularly in mares with histories of repeated antibiotic treatment.
• Pseudomonas aeruginosa: 2.0% and Staphylococcus aureus: 1.5% — less common but clinically significant, particularly Pseudomonas which carries inherent antibiotic resistance challenges.

What the Streptococcal Dominance Means in Practice

The fact that beta-haemolytic streptococci account for nearly 80% of positive cases is not a surprise to equine reproductive specialists — but seeing it confirmed across nearly 29,000 samples is important for treatment prioritisation.

Streptococcus equi subsp. zooepidemicus (“Strep zoo”) is not just the most common uterine pathogen — it is also the pathogen most associated with the dormant persister phenomenon. Research by Prof. Anders Miki Bojesen and Dr. Morten Rønn Petersen has demonstrated that Strep zoo is uniquely capable of entering a dormant state within the endometrial mucosa, where it:

• Evades detection by standard uterine swab culture
• Survives routine antibiotic treatment
• Persists across breeding seasons
• Reactivates during oestrus, generating the chronic low-level inflammation that blocks embryo implantation

This means that the 79.7% streptococcal prevalence in the German study represents detectable streptococcal infections only. A significant additional proportion of mares will carry dormant streptococcal infections that did not produce a positive result on the day of sampling.

Implications for Diagnostic Strategy

The German study’s data reinforces the limitations of relying solely on swab culture as a diagnostic tool. A single negative swab in a mare with a history of infertility cannot rule out streptococcal endometritis — it can only confirm that no active bacteria were present at that sampling moment.

More precise diagnostic approaches that account for dormant infection are needed. bActivate addresses this directly by providing a method to reactivate dormant bacteria before sampling, ensuring that the culture result reflects the true microbiological status of the uterus rather than a potentially misleading snapshot.

The Antimicrobial Resistance Dimension

The study also raises important questions about antimicrobial stewardship in equine reproduction. When broad-spectrum antibiotics are applied empirically — without a confirmed diagnosis — they eliminate active bacteria while dormant cells survive. The selective pressure this creates favours resistant strains, and the cycle of infection continues with potentially more resistant pathogens.

Targeted treatment based on a confirmed post-activation culture, as used in the bActivate protocol, supports more responsible antibiotic use by ensuring that treatment is guided by an accurate diagnosis rather than empirical assumption.

Moving Forward

The German study’s findings serve as both a description of the current challenge and a call for diagnostic improvement. Understanding that streptococcal infection is ubiquitous in the broodmare population — and that dormant forms are systematically underdetected — changes how we should approach the problem mare.

For breeders and veterinarians managing mares with unexplained infertility, these data provide scientific grounding for pursuing dormant infection as a diagnosis. Read about the clinical indicators for bActivate use or review the field study outcomes.

Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT.